It is generally known that a large excitation inrush current may flow when non-load excitation is performed by powering on a transformer while the core of the transformer has a residual magnetic flux. The magnitude of this excitation inrush current is several times as large as the rated load current of the transformer. The voltage of a power system fluctuates if a large excitation inrush current like this flows. If this voltage fluctuation is large, it may have an influence on users.
As a typical method of suppressing the above-mentioned excitation inrush current, a method in which a breaker is placed in a path for powering on a transformer is known. This breaker can make a contact with a resistor obtained by connecting an input resistance in series with a contact. The breaker is so configured as to suppress an excitation inrush current by connecting the contact with a resistor in parallel to a main contact, and turning on the contact with a resistor earlier than the main contact.
On the other hand, a directly grounded three-phase transformer is sometimes powered on via three single-phase breakers. In this case, a method of suppressing an excitation inrush current by first closing one arbitrary phase and then closing the two remaining phases is already known. Also, as a method of suppressing an excitation inrush current when powering on a three-phase transformer installed in a power system by using a three-phase integrated type breaker, a method which measures the value of a magnetic flux remaining in the core when the transformer is shut down and controls an excitation inrush current generated by power-on of the transformer by using the close phase of the breaker is known.
When supplying power to a single-phase AC apparatus such as a single-phase electric furnace or single-phase AC vehicle, a transformer obtained by a Scott connection, Woodbridge connection, modified Woodbridge connection, or roof-delta connection is used in order to convert a three-phase AC voltage into a single-phase AC voltage. By contrast, the excitation inrush current suppressing method using the breaker having the contact with a resistor requires the addition of the contact with a resistor when compared to an ordinary breaker. Consequently, the size of the whole system increases. The target of any other method is a three-phase transformer, and each method assumes a transformer which converts a three-phase AC voltage into a single-phase AC voltage as described above. Accordingly, none of these methods can be used.
It is, therefore, an object of the present invention to provide an excitation inrush current suppressing apparatus and excitation inrush current suppressing method capable of suppressing an excitation inrush current generated when connecting a transformer which converts a three-phase AC voltage into a single-phase AC voltage to a single-phase AC-side power system.